Physical conditions, structure, and dynamics of a zoned magma chamber: Mount Pelée (Martinique, Lesser Antilles Arc)

Experimental results and petrologic observations of the eruptive history of Mount Pelée are integrated, and a model for the magma storage system is presented. Recent (stage 3) Plinian and Pelean activity (P1, 650 years B.P.; 1902, 1929) erupted relatively homogeneous andesites (average 62 wt % SiO 2 ). They are porphyritic (35–58 vol % crystals) and contain phenocrysts of plagioclase (Plag) (An 50–90 ), orthopyroxene (Opx) (En 52–60 ), and magnetite (Mt) (∼Mt 70 ). Glasses (both interstitial and trapped) are rhyolitic (74–77 wt % SiO 2 ). Clinopyroxene (Cpx), ilmenite (Ilm), amphibole (Amph) (mostly resorbed pargasitic hornblendes), and olivine (Ol) are present as minor phases. Products of 1902 and 1929 contain mafic enclaves (51–59 wt % SiO 2 ) with compositions similar to basaltic andesite lavas erupted during stage 2 (40,000–19,500 years B.P.). Conditions in the andesitic part of the magma chamber, as determined from experimental phase equilibria, do not differ between the P1, 1902, and 1929 eruptions (875–900°C, 2 ± 0.5 kbar, ΔNNO = +0.4–0.8, melt H 2 O content of 5.3–6.3 wt %). New experimental data on a basaltic andesite composition (53 wt % SiO 2 ) from stage 2, at 4 kbar, 950–1025°C, for melt H 2 O concentrations from 8.3 to 2.6 wt %, and f O2 between NNO and NNO + 4 simulate crystallization in the mafic part of the chamber. Liquidus or near‐liquidus Ol, An‐rich Plag, Al‐ and Fe 3+ ‐rich salite and augite, pargasitic hornblende, and Al‐ and Mg‐rich Mt have compositions close to phenocrysts in mafic products from stages 2 and 3. Experimental liquids range from basaltic andesite to dacite. Application of experimentally derived mineral‐melt Al/Si and Fe/Mg partition coefficients to mineral compositions from mafic lavas and cumulates from stage 2 shows that the chamber is fed by relatively evolved parental basaltic liquids (Mg # ∼ 55–60)(Mg # = Mg/(Mg + Fe T ). They have low temperatures (≤1050°C), high melt H 2 O contents (>5–6 wt %), and f O2 (ΔNNO mostly between +1 and +2) and crystallize an Ol + Cpx + Mt assemblage followed by Plag + Amph, although Amph may have started to crystallize with Ol and Cpx. Compositions of natural glasses and amphibole in mafic cumulates and lavas record a continuous evolution from basaltic‐basaltic andesite to basaltic andesite‐dacite liquids. Crystal fractionation of basaltic magmas is the main process controlling the chemical diversity at Mount Pelée. Crystallization in the mafic part produces an andesitic‐dacitic residual liquid which subsequently evolves to produce the andesitic part. The present‐day situation is typical of low fluxes of mafic magmas in comparison with stage 2.